Ultrasonic imaging utilizing high frequency ultrasound of 12 mHz in combination with constructive interference to increase the resolution of reflected images.
The invention of medical diagnostic ultrasonography itself was not due to the invention of a unique technique, but rather is a new application of existing knowledge. SONAR had been invented a decade earlier. In the early 1950""s, Ehlers and Hertz of Sweden took an industrial ultrasound device xe2x80x9coff-the-shelfxe2x80x99 and tried it out on a human subject. Fortuitously, the ultrasound frequency of the device happened to be an appropriate one for humans, so that they were able to make a recording of the heart""s mitral valve. All of the thousands of subsequent developments in medical ultrasonography emanate from here.
Sound can be represented as a sine wave caused by propagation of alternative phases of molecular compression and rarefaction. Sound has two important characteristics: intensity (loudness) and frequency.
The ultrasound beam progressively loses part or all of its energy as it travels through space and objects, a process called attenuation.
Ultrasound beams are not concordant, i.e., the crests and troughs of each of their constituent sub-beams are randomly distributed in time. As a result, many crests partially or completely are overlapped by troughs, thereby partially or completely canceling them out. Consequently, reflected beams are much weaker than they could be if they were concordant. Thus, discordancy results in decreased image resolution.
A medical ultrasound machine consists of two components: a small, freely mobile, hand-held transducer, and a large, but portable, console. The transducer is that device which is held against the skin or other accessible body surface of the patient to send and receive the ultrasound signals. The console processes the ultrasound signals by various kinds of complex electronic circuitry, for real time display on a cathode ray screen, and on video printer paper for detailed analysis and hard copy permanent record keeping, The transducer is tethered to the console by a flexible electric cable.
The transducer includes a circular (or other) array of multiple ultrasound transmitters (crystals) which create (and receive) converging ultrasound beams at the target organ, just like the ribs of an umbrella converge at the umbrella""s distal end. The depth of convergence (focus) is determined manually (steered) by the examining physician or technologist, depending on the depth of the organ being investigated, and the superimposition of the crests is done automatically by a computer.
Medical ultrasonography utilizes reflected (echo) ultrasound, rather than transmitted ultrasound. In this way, it is like radar, and unlike x-ray radiography. Ultrasound, like electromagnetic radiation, undergoes transmission, absorption, scatter and reflection in the body. Attenuation results from absorption and scatter. Medical ultrasound transducers incorporate both the transmitting and receiving crystals. There are no biological ill-effects from medical ultrasonography, unlike x-ray studies.
The higher the ultrasound frequency, the greater the detail (resolution) of the image, but the greater the attenuation, and the less the depth of penetration. As a result, deep structures are visualized only faintly or not at all by high frequency ultrasound transducers.
On the other hand, lower frequency ultrasound can penetrate deeper, but its images lack fine detail (poorer resolution).
Because of the higher degree of attenuation of high frequency sound and the lower resolution of low frequency ultrasound, sound imaging of deep structures in the human body, like the heart and most abdominal organs, and particularly in large people, is not capable of providing the maximum degree of clarity desired by medical diagnosticians.
Current medical diagnostic ultrasound systems use low fundamental carrier frequencies in the range of 2.0-3.5 mHz to visualize the deep-lying viscera of the chest, abdomen, and pelvis. In recent years, manufacturers have used computer amplification to capture and record the second harmonics of echoing ultrasound beams in order to increase image resolution.
Theoretically, the penetrability of high frequency ultrasound could be increased by increasing its energy (loudness) just by turning up the wattage. Unfortunately, such high energy, high frequency ultrasound can cause skin bums and chromosome damage in the skin and/or internal organs. Hence, this option is not available for use in biological systems.
Digitization, the use of color rather than a gray scale, intravenous contrast agents, and other techniques also recently have been introduced to improve image resolution.
The only other known way to increase ultrasound penetration is by the recently described SASER. This device is the sound counterpart of the LASER, and depends on magnifying sound by a kind of ricocheting chain-reaction similar to what happens to light in the LASER machines. If it worked well, it too would be expected to cause skin burns and chromosomal damage for the same reasons as noted with respect to high energy, high frequency ultrasound. To date, it does not work that well, magnifying sound only up to 50%. Furthermore, present SASERs require temperatures far below zero. Hence, SASERs do not have any biological applicability at present.
LASERS and MASERS use constructive interference of light and microwaves respectively for their own special purposes. Similarly, noise cancellation technology patents employ sound destructive interference for their specific application.
Johnson, U.S. Pat. No. 4,222,274, dated Sep. 16, 1980, entitled Ultrasonic Imaging Apparatus and Method, the disclosure of which is incorporated herein by reference, is an early patent which discloses an ultrasound imaging apparatus which utilizes the principle of constructive interference to enhance the signal strength of a reflected wave. In this particular patent, constructive interference is obtained by transmitting a particular waveformxe2x80x94which has the characteristic of a high amplitude center lobe with symmetrical low amplitude side lobes.
Fox, U.S. Pat. No. 4,307,613, dated Dec. 29, 1981, entitled Electronically Focused Ultrasound Transmitter, discloses a transmitter for use in an ultrasound apparatus that comprises an annular array of transducer elements which are arranged such that the emitted acoustic waves constructively interfere at an electronically controlled focal point. This patent is not concerned with the reflected wave, but does teach the benefit of constructive interference for enhancement of the transmitted wave at the point to be imaged. It also teaches the use of an annular array having multiple transducers.
Perhaps the best exposition of the utilization of constructive interference for the enhancement of signal strength is found in Knuttel, et al., U.S. Pat. No. 5,203,339, dated Apr. 20, 1993 and entitled Method and Apparatus for Imaging a Physical Parameter in Turbid Media Using Diffuse Waves. Knuttel discusses the benefits of constructive interference appurtenant to the use of light photons/waves, and goes into some detail regarding improved signal resolution without the need to increase the amplitude of the transmitted wave.
There are other patents relate to the benefits of constructive interference in enhancing transmitted or received acoustic waves.
In sum, the broad idea of utilizing constructive interference in an ultrasound imaging apparatus is well known.
Several companies already manufacture noise-dampening devices based on the reverse of the amplification principle described above. The devices emit sound which is synchronized by digital computers to be out of phase with the noise, so that their respective crests and troughs overlap and thereby cancel themselves out.
Diagnostic medical ultrasound equipment was first invented in the 1950s. Many advances have been made since then. Many current medical diagnostic ultrasound systems employ the principle of constructive interference with low frequency ultrasound transducers to help improve the rather poor image resolution of deep lying structures when interrupted by low frequency ultrasound.
However, no current technique has been able to achieve the degree of resolution desired by and necessary to bring the full fruits of ultrasound imaging to medical diagnostics. For unknown reasons, no one has thought to combine constructive interference with high frequency ultrasound to achieve a unique combination of advantages not otherwise obtainable in medical imaging.
Thus, there exists a continuing need for a high resolution ultrasound diagnostic system that is uncomplicated and simple to use.